Microorganisms and the slimes they produce are responsible for the formation of deposits in papermaking and industrial cooling water systems. Bacterial slimes are composed of exopolysaccharides (EPS) which exist as capsules or slime layers outside of the cell walls. When these slimes form on surfaces in paper or cooling systems, they trap organic and inorganic components and debris present in the process waters. As the microorganisms grow within paper system deposits, portions of the deposit may detach from the surface and cause paper breaks and spots in produced paper, which reduces the paper quality and increases machine downtime. Microbial growth and slime formation in cooling systems results in reduced heat exchange caused by biofouling and plugging of heat exchanger tubes, excessive fouling of the cooling water, tower decks and fill, and is a potential cause of under-deposit corrosion.
The term "slime" is a broad one covering a wide range of viscous, mucous, or leathery materials and mixtures found in industrial waters. Slimes are polymeric in nature and can be broadly classified as chemical, biological, or composite slimes depending upon their cause or composition. For example, raw materials and equipment used in the paper industry are not sterile and water used in conjunction with such equipment is continuously being contaminated with a wide variety of micro organisms from such sources as wood pulp, chemicals, air, makeup water, and the like. The growth of certain specific forms of these biological contaminants causes or produces polymeric excretions or products that are or become slime.
Historically, slime formation has been treated by the addition to industrial waters (e.g., white water associated with the pulp and paper industry) of slimicides. The purpose of these slimicides is to destroy or arrest the growth of some of the many organisms present in the water to thereby prevent or retard the formation of slime. Chemicals used as slimicides have included chlorine, phenylmercuric acetate, pentachlorophenol, tributyl tin oxide, and isothiocyanates, all of which are relatively toxic to humans.
Microbially produced exopolysaccharides can build up, retard heat transfer and restrict water flow through cooling water systems. Controlling slime-forming bacteria by applying toxic chemicals is becoming increasingly unaccepted due to environmental problems. In addition, the efficacy of the toxicants is minimized by the slime itself, since the extracellular polysaccharide surrounding microorganisms impedes toxicant penetration.
Toxicants cannot adequately control large populations of attached bacteria and they are effective mainly against floating microorganisms. Although surfactants and dispersants which penetrate and help loosen slime can enhance the activity of toxicants, they are nonspecific and may have deleterious effects on the industrial process or the environment.
Recently, methods directed at controlling microbial slimes include the use of enzymes. These approaches attempt to disrupt the attachment process so that slime formation is prevented, or by hydrolyzing the exopolysaccharide (EPS) produced by the microorganisms after attachment. Using an enzyme to control slime will require knowledge of the composition of the slime, so that an appropriate enzyme-substrate combination is employed.
Differing views of the composition of industrial slime deposits exist, but no data directly supporting those views have been published. Research by H. J. Hatcher (Biochemical Substances as Aids in Process Control, TPPI 62(4):93, 1980) suggests that slimes are composed of levan, a homopolysaccharide composed of repeating units of fructose. This is inconsistent with literature related to the biosynthesis of Tevans, which shows that levans can only be produced by bacteria growing on sucrose (Stanier, R. Y. E. A. Aelber, and J. L. Ingraham; Structure and Function in Procaryotic Cells, Capsules and Slime Layers, The Microbial World. pp. 335-337, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1975). During levan biosynthesis, the fructose unit of sucrose is incorporated into levan, while the glucose unit is used for energy and growth by the microorganism. It is unlikely that bacteria in paper or cooling systems will encounter sucrose in significant amounts, and hence levans should not be a significant component of industrial slimes.
A recent Canadian Patent, No. 1,274,442, suggested that EPS of industrial water systems may be composed of alginate, a polysaccharide composed of mannuronic and guluronic acids. U.S. Pat. No. 4,936,994, claimed that microbial slime in industrial systems can be controlled using cellulase, .alpha.-amylase, and protease, which presupposes that the slimes result from polymers of .alpha.-and .beta.-linked glucose residues and protein.
These differing views can be contrasted with those more commonly accepted in the technical literature, that slime-producing bacteria form heteropolysaccharides consisting of monosaccharides such as glucose, mannose, galactose, and glucuronic acid. Studies have shown that the composition of EPS is usually independent of the substrate or carbon source used for growth and polysaccharide production (Sutherland, I. W. Bacterial Exopolysaccharides-Their nature and Production, Surface Carbohydrates of the Procaryotic Cell (Sutherland, I. W., ed), pp, 27-96, Academic Press Inc. Ltd., London, 1977). It was therefore felt that slimes found in industrial cooling and papermaking systems were likely to be composed of mixed heteropolysaccharides.
It is an object of the present invention to provide a microbial slime treatment program which will more effectively break down or altogether prevent the production of microbial exopolysaccharide slime than any of the other treatment approaches heretofore employed. This is achieved by using a combination of enzymes that are specific to each of the saccharides present in the slime layer.